1. Field of the Invention
The present invention relates to a current detection circuit for detecting current flowing in a light receiving device and a light emission controller for controlling light emission of a light emitting device.
2. Description of the Related Art
In various kinds of electronic equipment, light incident from outside is measured and signal processing in response to the measured amount of received light is performed. As an example, a light receiver such as an illuminance sensor and an infrared remote control is cited. As a light receiving device for receiving light, a phototransistor, photodiode, CCD (Charge Coupled Device), and so on are widely used.
The phototransistor and the photodiode output current in response to the amount of received light. Therefore, the light receiver performs signal processing by amplifying or voltage-converting the current flowing these light receiving devices. For example, in Patent Document 1, a light receiver as an infrared remote sensor is disclosed and a configuration of a light receiving circuit is disclosed in FIG. 3 thereof. Further, in Patent Document 2, a configuration of a light receiving circuit using an operational amplifier is disclosed.
Also, in various kinds of electronic equipment, light incident from outside is measured and signal processing in response to the measured amount of received light is performed. As an example of the signal processing, light emission control of a camera flash is cited. Specifically, light emitted from a flash and reflected by a subject is measured, and, when the reflected light reaches a predetermined level, the flash (hereinafter, also referred to as a light emitting device) is turned off. In Patent Document 3, a related technology is described.    [Patent Document 1] Japanese Patent Application Laid-open No. H6-188835    [Patent Document 2] Japanese Patent Application Laid-open No. 2005-216984    [Patent Document 3] Japanese Patent Application Laid-open No. 2005-10366
Problem 1
In such a light receiver, there are cases where the amount of received light is desirably detected only with predetermined timing. As an example, the emission control of a flash in a camera or cellular phone terminal with camera will be considered. Regarding the light emission time of the flash, control is performed in a manner that the light illuminated by the flash on and reflected from a subject to be imaged and returned is detected and, when a predetermine amount of light is reflected, the light emission of the flash is stopped. Accordingly, if the detection of light is performed before the flash emits light, sunlight or fluorescent light that should not be measured are also measured, and thus, a problem that accurate light emission control can not be performed arises.
Problem 2
Further, in such equipment, the case where a light receiving device such as a phototransistor or photodiode for generating photocurrent in response to the amount of received light is used for receiving reflected light will be considered. To perform the above-described light emission control using these light receiving devices, a technique of integrating the photocurrent and stopping the light emission when it reaches a predetermined value is conceivable. Here, as a technique of easily integrating photocurrent, there is a method of flowing photocurrent in a capacitance for charging. In this case, the electric potential appearing in the capacitance is compared to a threshold voltage and the light emitting device is turned off according to the comparison result.
However, after the potential appearing in the capacitance reaches the threshold voltage, sometimes the potential drops due to discharge or the like or varies by the influence of noise. In this case, when the potential appearing in the capacitance drops below the threshold voltage, the light receiving device emits light again. Then, when the potential appearing in the capacitance by the re-light-emission reaches the threshold voltage, the light emission is stopped again, and thus, the emission and non-emission may be repeated.